The invention relates generally to plasma sputter reactors. In particular, the invention relates to complexly shaped sputter targets.
Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. Most commercial sputter reactors rely upon magnetron sputtering in a plasma reactor. The most common commercial sputter reactor is a magnetron sputter reactor in which a metal target of the material to be sputter deposited is placed in opposition to the wafer to be sputter coated. The vacuum chamber containing the wafer and target is filled with a few milliTorr of argon. The target is then electrically biased to a few hundred volts DC, which excites the argon into a plasma. The resulting positively charged argon ions are attracted to the negatively biased target and dislodge (sputter) metal atoms from the target. Some of the metal atoms fall on the wafer and coat a thin metal layer on it. Typically, a set of magnets, called a magnetron, is placed in back of the target to create magnetic field lines parallel to the front face of the target, thereby trapping electrons and increasing the plasma density adjacent the target and thus increasing the sputtering rate. In reactive sputtering, a reactive gas such as nitrogen is also admitted to the chamber, and the reactive gas reacts with the sputtered metal atoms to form a metal compound, such as a metal nitride, on the wafer surface.
The older, conventional magnetron sputter reactors produce a relatively low-density plasma of the argon ions and, as a result, the sputtered metal atoms are mostly neutral, only a few percent of them being ionized. It has become recognized in recent years that a higher fraction of metal ions would be very beneficial, particularly for coating the sides and bottoms of holes having high aspect ratios. Such holes may be via or contact holes or may be DRAM trenches. The mostly ballistic sputtering process described to this point is ill suited for reaching into holes having aspect ratios significantly larger than one at the same time that vias of modern integrated circuits often have aspect ratios of 5 and greater. However, it has been recognized that a negatively biased wafer can accelerate metal ions in the direction normal to the wafer surface, thereby draw the sputtered metal ions deep into the hole.
Generally, increasing the density of the argon plasma increases the ionization fraction of the sputtered atoms. Several approaches have been used to produce a high density plasma. In one approach, additional RF energy is inductively coupled into a plasma source region remote from the wafer. In a second approach, often called a hollow cathode reactor, a non-planar target surrounds the top and sides of a plasma region adjacent the target, thereby reducing the plasma loss and increasing the plasma density. In a third approach, often called self-ionized plasma (SIP) sputtering, a small intense magnetron concentrates the target power in a reduced area, thereby increasing the power density and hence increasing the plasma density adjacent to the magnetron. The small magnetron is scanned around the target to produce more uniform sputtering.
An advanced sputter reactor that advances on the second and third approach is the SIP+ sputter reactor marketed by Applied Materials, Inc. of Santa Clara, Calif. and schematically illustrated in FIG. 1. Reactors of this type have been described by Gopalraja et al. in U.S. Pat. No. 6,277,249 and U.S. patant application, Ser. No. 09/703,601, filed Nov. 1, 2000 and now issued as U.S. Pat. No. 6,451,177, both of which are incorporated by reference herein in their entireties. The lower part of the reactor 10 includes an electrically grounded chamber including sidewalls 12 generally symmetric about a central axis 14. A vacuum pumping system 16 reduces the base pressure within the chamber to the neighborhood of 10xe2x88x928 Torr. However, working gas is supplied from an argon source 18 through a mass flow controller 20 to maintain the argon pressure in a range of 0.1 to 10 milliTorr. If a nitride film is being formed by reactive sputtering, nitrogen is additionally supplied.
A wafer 22 to be sputter coated is supported on a temperature controlled pedestal electrode 24. The wafer 22 may be secured to the pedestal electrode 24 by a clamp ring 26, but an electrostatic chuck may alternatively be used. A grounded shield 28 supported on the sidewalls 12 protects the chamber walls and sides of the pedestal 24 from being coated with sputtered material and further acts as a cathode for the diode sputtering process. The argon working gas is admitted into a processing space 30 over the wafer 22 through gaps between the pedestal 24, the wafer clamp 26, and the grounded shield 28. The high density plasma being generated benefits from an electrically floating shield 32 supported on the grounded shield 28 through an isolator 34.
The SIP+ reactor 10 is most visibly distinguished by a target and magnetron assembly 40 including a vault-shaped target 42 supported on the chamber sidewalls 12 through a second isolator 43. The target 42 is composed of the metal to be sputtered. Copper sputtering is the most prevalent initial use of the SIP+ reactor 10, but other metals can be used in the target 42. The vault-shaped target 42 includes an annular vault 44 extending around the central axis 14 with its open end or throat facing the wafer 22. The vault 44 includes an outer sidewall 46, an inner sidewall 48, both extending generally parallel to the central axis 14, and a roof 50 extending generally perpendicular to the central axis 14. A central well 52 is formed on the back of the target 42 inside the annular inner sidewall 48. The target 42 is supported on the isolator 43 by an outwardly extending flange 54. A projection 56 extending downwardly from the outer sidewall 46 forms a plasma dark space in opposition to the floating shield 32.
A DC power source 58 electrically biases the target 42 to a negative voltage of about xe2x88x92600 VDC with respect to the grounded shield 28. This voltage is sufficient to maintain an argon plasma within the processing space 30. If a substantial fraction of the sputtered atoms are ionized, it is advantageous to induce a negative DC bias on the pedestal electrode 24 by biasing it with an RF power supply 60 connected to the pedestal electrode 24 through an unillustrated capacitive coupling circuit. A controller 62 controls the sputtering process and may be programmed for a multi-step process according to which it separately controls the chamber pressure, target power and wafer bias.
In magnetron sputtering, magnets are positioned in back of the target 42 to increase the plasma density adjacent to the face of the target 42. The SIP+ target and magnetron assembly 40 includes both stationary and rotating magnetic parts. The stationary part includes a large number of permanent magnets 70 of a first vertical polarity arranged around the outside of the outer vault sidewall 46. A cylindrical magnet 72 of an opposite second vertical polarity is disposed within the vault well 52 behind and inside the vault inner sidewall 48. Although the cylindrical magnet 72 is rotating for reasons relating to unillustrated target cooling, its magnetic field is essentially stationary. The two sets of magnets 70, 72 create anti-parallel magnetic fields close to interior sides of the vault 44 adjacent the opposed sidewalls 46, 48. The rotating part includes a nested magnetron 74 positioned over the vault roof 50 and including an outer annular magnet 76 of the first magnetic polarity surrounding an inner cylindrical magnet 78 of the second magnetic polarity. The nested magnetron 74 is unbalanced in that the total (spatially integrated) magnetic flux produced by the outer magnet 76 is at least 50% larger than that produced by the inner magnet 78.
The roof magnetron 74 is supported on a magnetic yoke 80 fixed to a shaft 82 extending along the central axis 14 and rotated by an unillustrated motor so as to sweep the roof magnetron 14 along the circumference of the roof 50 of the target vault 44. The inner sidewall magnet 72 is also supported through a non-magnetic spacer 84 connected to the shaft 82 although this rotation is not immediately pertinent to the physics of the sputtering process.
The described magnetron in conjunction with the annularly vaulted target offers many advantages. The vault creates a region closed on three sides so that plasma leakage out of the sputtering region is minimized and the plasma density is increased. The magnetic field components running parallel to the target sidewalls 46, 48 and to the roof 50 further increase the plasma density near the target areas being sputtered. The relatively small roof magnetron 74 concentrates the sputtering in the area of the vault 44 over which the roof magnetron is passing, thus concentrating the limited target power there and increasing the target power density. Sputtering into high aspect-ratio holes is facilitated by a large fraction of ionized sputtered metal particles which can be attracted into the holes by biasing the wafer. The SIP+ reactor is believed to be capable of a metal ionization fraction of about 50%. The combination of a stationary distributed magnetic field and a rotating localized magnetic field allows the magnetron to operate in two distinct sputtering modes, believed to be associated with sputtering around the entire annular vault and with sputtering in the area of the roof magnet.
Nonetheless, the SIP+ reactor could be further improved. In at least some applications, particularly those involving extreme aspect ratios of ten and more, it is desired to further increase the ionization fraction since any neutral sputter component is approximately isotropic, a cosine distribution off the normal between the target and wafer being assumed. As mentioned before, SIP+ sputter reactors as presently implemented seem to be limited to about a 50% metal ionization fraction. The ionization rate in SIP+ reactors is practically limited by the plasma density produced by the still relatively low target power. The localized sidewall and magnetic field confinement still allows excessive plasma leakage from the high-density plasma region.
Although SIP+ targets provide relatively good sputtering uniformity, the sputtering uniformity on sidewalls across the wafers could be improved. The geometry of the target 42 and the wafer 22 with its high aspect-ratio holes 90 is illustrate in the cross-sectional view of FIG. 2 The holes 90 will hereafter be referred to as vias because this type of vertical connection through an inter-level dielectric between two metallization levels is a major application. The thickness of the wafer 22 is greatly exaggerated, but the geometry of the vias 90 is approximately correct. It has been observed that the minimum target erosion occurs at the outer vault sidewall 46. That is, the greatest sputtering rate occurs at the outer sidewall 46. In the usual configuration, the diameter of the wafer 22 generally extends from appoximately one side to the other of the middle of the annular vault 44. If the vias 90 are located near the periphery of the wafer 22, this geometry exposes the hole inner sidewall 92 of the full brunt of the target sidewall sputtering. As a result, the inner hole sidewall 92 is subject to a larger flux of neutral target atoms than is the outer hole sidewall 94. This differential flux tends to form an overhang 98 on the inside hole rim 96, which has the possibility of closing off the hole 90. Other geometries may favor inner sidewall deposition. Sidewall coverage is critical for formation of a thin copper seed layer. To minimize seed deposition times and prevent premature via closure, the sidewall deposition should be uniform across the wafer.
The annularly vaulted target is related to a well known hollow-cathode target, for example, that described by Lai et al. in U.S. Pat. No. 6,193,854 and by Lai in U.S. Pat. No. 6,217,716 although significant differences exist in both the geometry and effect of the magnetic fields. Such a hollow-cathode includes a single cylindrical vault arranged about the chamber axis and having a tubular sidewall and a roof bridging the sidewall. The cited references describe several magnet configurations. The hollow cathode is in turn related to an effusion cell or partially closed hollow sputtering target, such as that described by Glocker in U.S. Pat. No. No. 5,069,770. In the effusion cell, the throat of the cylindrical hollow target facing the wafer is partially closed with a narrow opening at its symmetric center facing the wafer. The effusion cell can be likened to a black-body radiator in which an intense plasma develops within the cell""s interior with only a relatively small portion leaking through the central aperture towards the wafer. This geometry does not address the problem of sidewall uniformity. Glocker""s effusion cell is distinguished from a more conventional hollow cathode in that the effusion cell includes both an anode and a cathode within the target cavity.
A sputtering target having an annular vault arranged about its side facing the substrate being sputter coated. The throat of the vault is partially closed. The throat ring may be arranged adjacent the inner sidewall or the outer sidewall of the vault or adjacent both sidewalls so as to form a smaller annular throat. Alternatively, the throat ring may be formed with a circular arrangement of holes. The holes may be circular or circumferentially elongated, and they may be formed in multiple concentric circles.
The invention may also be applied to a hollow cathode target having a cylindrical vault.
The partially closed throat more effectively confines the plasma within the vault, thereby increasing the plasma density and the ionization fraction of sputtered atoms.